The present invention relates to a high pressure discharge lamp and a method for producing the same. In particular, the present invention relates to a high pressure discharge lamp used for general illumination or other applications such as projectors and headlights of automobiles when it is used in combination with a reflecting mirror.
In recent years, an image projection apparatus such as a liquid crystal projector and a DMD projector has been widely used as a system for realizing large-scale screen images, and a high pressure discharge lamp having a high intensity has been commonly and widely used in such an image projection apparatus. FIG. 14 is a schematic view showing the structure of a conventional high pressure discharge lamp 1000. The lamp 1000 is a so-called ultra-high pressure mercury lamp.
The lamp 1000 includes a luminous bulb 110 made of quartz glass, and a pair of sealing portions 120 (seal portions) extending from both ends of the luminous bulb 110. In the luminous bulb 110 (discharge space), a luminous material (mercury) 118 is enclosed, and a pair of tungsten electrodes (W electrode) 112 made of tungsten are opposed to each other with a certain gap. One end of the W electrode 112 is welded to a molybdenum foil (Mo foil) 124 in the sealing portion 120, and the W electrode 112 and the Mo foil 124 are electrically connected. An external lead (Mo rod) 126 made of molybdenum is electrically connected to one end of the Mo foil 124. In addition to the mercury 118, argon (Ar) and a small amount of halogen are enclosed in the luminous bulb 110.
Next, the operational principle of the lamp 1000 will be described. When a start voltage is applied to the W electrodes 112 and 112′ via the external leads 126 and the Mo foils 124, discharge of argon (Ar) occurs. Then, this discharge raises the temperature in the discharge space of the luminous bulb 110, and thus the mercury 118 is heated and evaporated. Thereafter, mercury atoms are excited and become luminous in the arc center between the W electrodes 112 and 112. As the pressure of the mercury vapor pressure of the lamp 1000 is higher, the lamp is more suitable as a light source for an image projection apparatus. However, in view of the physical strength against pressure of the luminous bulb 110, the lamp 1000 is used at a mercury vapor pressure of 15 to 25 MPa (150 to 200 atm).
The conventional lamp 1000 has a strength against a pressure about 20 MPa, but research and development to increase the strength against pressure have been conducted in order to improve the lamp characteristics further. However, a high pressure discharge lamp having a significantly high strength against pressure (e.g., about 30 MPa or more) for practical use has not been realized yet. Also there is a demand for a long life of a lamp and it is preferable that in the high pressure discharge lamp, blackening occurring in the luminous bulb 110 can be prevented effectively.